BUG/MINOR: ssl: ssl_pkey_info_index ex_data can store a dereferenced pointer

BUG/MINOR: ssl: ssl_pkey_info_index ex_data can store a dereferenced pointer

With CLI cert update, sni_ctx can be removed at runtime. ssl_pkey_info_index
ex_data is filled with one of sni_ctx.kinfo pointer but SSL_CTX can be shared
between sni_ctx. Remove and free a sni_ctx can lead to a segfault when
ssl_pkey_info_index ex_data is used (in ssl_sock_get_pkey_algo). Removing the
dependency on ssl_pkey_info_index ex_data is the easiest way to fix the issue.